This invention relates to a process for oxidizing fluorinated olefins to the corresponding epoxides and to the catalyst useful for the purpose.
More in particular, the present invention relates to a process for oxidizing tetrafluoroethylene and hexafluoropropene, and to the relevant catalyst.
2. The prior art
Processes for oxidizing fluorinated olefins, for example tetrafluoroethylene and hexafluoropropene to the corresponding epoxides, such as the processes described in U.S. Pat. Nos. 3,775,438, 3,775,439 and 3,775,440, are known in the art.
According to what is described in this literature, hexafluoropropene and tetrafluoroethylene are oxidized with oxygen, either pure or mixed with inert gases, in the presence of a catalyst essentially consisting of silica.
The catalyst, which has a SiO.sub.2 content ranging from 60 to 95% by weight, is used in different forms, particularly as a gel, as ground glass, as macroporous beads or also as sand.
These processes, although they permit to obtaining sufficiently high yields and conversions to epoxides, have very evident limits, which render them little suited to be utilized on a commercial scale. One of these limits is due to the catalyst life: in fact the catalyst becomes disactivated unless it is continuously treated with water or steam.
Another limit is due to the fact that the oxidation reaction, under certain conditions, requires a higher pressure than the atmospheric pressure or the use of high temperatures and in both cases the drawbacks are not negligible: in the former case, the hazard, in the latter case, the risk of using a temperature close to the epoxide decomposition temperature and, by consequence, of reducing the yield of finished product.
Subsequently, other processes for preparing in particular hexafluoropropene oxide have been proposed and are described in the published Japanese patent application Nos. 77/53804, 77/53805 and 77/53806.
Like in the other cases, oxidation occurs by direct contact between fluoroolefin and catalyst, but the catalyst is composed of a product based on silica, which carries promoters selected from transition metals, such as copper, chrome, manganese, iron, zinc, palladium and cerium.
Also in such cases, however, the shortcomings which highly limit the described processes, mainly as regards conversions and reaction yields, are still existing. The corresponding values are very low also owing to the too high reaction temperatures generally ranging from 250.degree. to 300.degree. C.